This invention relates to optical beam combining arrangements and particularly to beam combiner members which merge the dual beams of a spectrophotometer into a composite beam along an axis. In a dual beam spectrophotometer, sample and reference beams of radiant energy are directed from separate substantially equivalent optical paths to a photoelectric detector where the intensity of each beam is measured. For the paths of the beams to be optically equivalent, the absorbing and reflecting characteristics of the optical elements in those paths must be matched. In prior art beam combining arrangements, substantial optical equivalence is only made possible by moving mirrors which then introduce mechanical and phasing problems. Furthermore, photoelectric detectors respond differently to beams making different incident angles thereon and this presents a problem with prior art beam combining arrangements which do not coaxially merge the beams into a composite beam. Otherwise, very expensive off-axis aspheric mirrors must be utilized in prior art beam combining arrangements to avoid image aberrations when the beams being combined are divergent.